Oropharyngeal squamous cell carcinoma (OPSCC), unlike other head and neck subsites, is increasing in incidence at 3-4% per year due to human papillomavirus (HPV) infection. After platinum-based concurrent chemoradiation therapy (platinum CRT), a current standard of care having significant morbidity, HPV(+) cases have better prognosis than HPV(-) cases. Nevertheless, both good-outcome HPV(-) and poor-outcome HPV(+) OPSCC occur. Current identification of good-prognosis patients for testing de-intensified treatments is based on clinical response to induction chemotherapy, with its associated risks and definitive treatment delay. Better tumor-associated objective biomarkers would be preferable for matching patients to effective treatments, whether intensified treatment for those at high risk of recurrence or studies of de-intensified treatment for those with better prognosis. We recently identified high expression of the anti-apoptotic protein Bcl2 as a marker of worse outcome for OPSCC patients treated with platinum CRT, independent of HPV status. We identified two groups with highly uniform outcomes: almost all OPSCC patients with Bcl2(-)/HPV(+) tumors are cured following platinum CRT, while Bcl2(+)/HPV(-) tumors uniformly do poorly. This project takes several crucial steps toward bringing categorization of OPSCC by Bcl2 expression and HPV status into clinical practice. We will prospectively validate this classification on 400 newly presenting patients, sharpening the estimates of the associated hazards. To enhance the classification of intermediate-prognosis cases, we will explore two biomarkers related to the treatment resistance function of Bcl2. First, we will use BH3 profiling technology, an established method to assess Bcl2 functional status, for the first time as a biomarker in OPSCC. We predict that Bcl2 functional status will explain outcome differences following platinum CRT among OPSCC classified as Bcl2(+) by immunohistochemistry. Second, Bcl2 interacts with the p53 tumor suppressor in several ways, suggesting that loss-of-function mutations in p53 will complement the Bcl2 expression biomarker. We predict that p53 mutational status will improve the classification of our intermediate- prognosis patients into high- and low-risk subsets. We will also test our hypothesis about the function of Bcl2 in treatment resistance, in an in vivo preclinical model based on xenografts derived from Bcl2(+) OPSCC, by determining whether BH3 profiling predicts xenograft response to the cisplatin used in current therapy and to the small-molecule Bcl2 inhibitor ABT-737. The high levels of Bcl2 associated with treatment resistance of OPSCC to platinum CRT provide a promising target for future directed therapy with Bcl2 inhibitors. This project will improve classification of OPSCC with respect to expected outcomes, based on biomarkers related to Bcl2 functional status and mechanisms of treatment resistance. It thus will support development of intensified treatments for those at high risk and of treatments with less morbidity for those at low risk.